Higher temperature, increased CO2, and changing nutrient ratios alter the carbon metabolism and induce oxidative stress in a cosmopolitan diatom

Abstract Phytoplankton are responsible for about 90% of the oceanic primary production, largely supporting marine food webs, and actively contributing to the biogeochemical cycling of carbon. Yet, increasing temperature and p CO 2 , along with higher dissolved nitrogen: phosphorus ratios in coastal waters are likely to impact phytoplankton physiology, especially in terms of photosynthetic rate, respiration, and dissolved organic carbon (DOC) production. Here, we conducted a full‐factorial experiment to identify the individual and combined effects of temperature, p CO 2 , and N : P ratio on the antioxidant capacity and carbon metabolism of the diatom Phaeodactylum tricornutum . Our results demonstrate that, among these three drivers, temperature is the most influential factor on the physiology of this species, with warming causing oxidative stress and lower activity of antioxidant enzymes. Furthermore, the photosynthetic rate was higher under warmer conditions and higher p CO 2 , and, together with a lower dark respiration rate and higher DOC exudation, generated cells with lower carbon content. An enhanced oceanic CO 2 uptake and an overall stimulated microbial loop benefiting from higher DOC exudation are potential longer‐term consequences of rising temperatures, elevated p CO 2 as well as shifted dissolved N : P ratios.